EC2 Master Discovery

This PR allows you to change the `master` setting in a minion config file to `ec2`, then add an additional stanza labeled `ec2_info` with some extra info as to what to search for inside EC2, and it should resolve an instance to use as a Salt Master automatically.  It can even allow you to emulate round robin DNS using no more than EC2 tags to some degree.

This is my first PR for Salt - let me know if I've messed anything up here or not followed any style guides/updated docs and I'll be sure to fix.

Thanks!